Such metal V-belts have been known, and some are disclosed in, for example, Japanese Laid-Open Utility-Model Publication No. S62(1987)-131143 and Japanese Laid-Open Patent Publications Nos. H2(1990)-225840 and H7(1995)-12177. As an example, a portion of one type of metal V-belt 13 is shown in FIGS. 5 and 6. This metal V-belt comprises metal rings 14, each of which is an endless belt, and a plurality of metal elements E, each of which is supported in succession along the metal rings 14. This metal V-belt is put around a drive pulley 11 and a driven pulley 12 and disposed therebetween, each pulley having a V-shaped groove, the width of which is variably adjustable. With this construction, when the widths of the V grooves of the pulleys are variably adjusted, the V-belt pitch radii of the pulleys are variably changed so that the speed ratio is continuously varied.
In designing a metal V-belt which can be used for a continuously variable transmission, it is important to improve the transmission efficiency of the metal V-belt. For this reason, the metal rings and the metal elements should be built with sufficient strengths while their sizes and weights are reduced as much as possible since metal V-belts are typically exposed to a rotational speed of up to 5,000 rpm or to 6,000 rpm. A method for reducing the weight of the metal element E is described in the above mentioned Japanese Laid-Open Utility-Model Publication No. S62(1987)-131143. In this method, the central bottom part (the bottom of the body portion 3 shown in FIG. 6) of the metal element E is cut off to reduce the weight of the metal element while the height (equivalent to the saddle primary thickness h2 in the following embodiment according to the present invention) of the remaining body of the metal element is regulated to attain a uniform pressure distribution over the V surface of the metal element, which surfaces of the metal elements are engaged with the V-shaped grooves of the pulleys.
Also, a method for improving the transmission efficiency of the metal V-belt is disclosed, for example, in the above mentioned Japanese Laid-Open Patent Publication No. H2(1990)-225840. The publication discloses a motion that removes the clearance among the metal elements in the slack side of the metal V-belt. In this method, each metal element is designed such that the center of gravity of the element is located near the inclined region on the main surface of the metal element or radially outside of the inclined region. With this construction, the metal element retains an impulsive force when it comes out of the driven pulley, and by this impulsive force, the metal elements in the return side of the metal V-belt are forced to connect with one another without clearance. As these metal elements in a successive connection without any clearance enter the V-shaped groove of the driven pulley, this metal V-belt drive has an improved efficiency of power transmission and a reduced friction.
Also, a method for improving the transmission efficiency and the abrasion resistance of the metal V-belt is disclosed, for example, in the above mentioned Japanese Laid-Open Patent Publication No. H7(1995)-12177. This method determines two contact points: a first contact point where the supporting surfaces (equivalent to the upper surfaces of the saddles 1 in FIG. 6) of the metal element are in contact with the innermost portions of the metal rings respectively, and a second contact point where the surfaces of a top portion (equivalent to the lower surfaces of the ears 6 in FIG. 6) of the metal element are in contact with the outermost portions of the metal rings respectively. Then, the metal V-belt is constructed to set the relation of the horizontal distance A between these two contact points with the clearance B in the direction of the thickness of the metal rings in the slots of the metal element as Tan.sup.-1 B/A is smaller than 1. With this construction, the magnitude of turning around and of translation (i.e., sliding) of the metal element at the entrance of a pulley is restricted to improve the transmission efficiency and the abrasion resistance.
However, the technique for reducing the weight of the metal element, which is described in the above mentioned Japanese Laid-Open Utility-Model Publication No. S62(1987)-131143 is to determine the contact length of the V surface of the metal element (hereinafter referred to as "element") which meets the V-shaped grooves of the pulleys and the height of the plate remaining after the central bottom part of the element has been cut off to make the distribution of the compressive stress over the V surface of the element uniform. However, this method determines the length of the V surface and the height of the plate not in consideration of the bending moment which results from the penetration of the element into the V-shaped groove of the pulley and which acts on the central portion of the element (i.e., the body 3 in FIG. 6).
However, when the plate thickness of the central portion of the element is determined to make the distribution of the compressive stress uniform over the V surface, even though this is effective in reducing the weight of the element, the element may be cut off excessively. As a result, the remaining portion of the element may be exposed to an increased bending stress, which may jeopardize the element to a fatigue failure. On the other hand, if the length of the V surface of the element is determined to make the distribution of the compressive stress uniform while the bending stress is kept sufficiently small, then the length of the V surface may become excessively great. As a result, the weight of the element may not be reduced sufficiently.
In the technique for improving the transmission efficiency which is disclosed in the above Japanese Laid-Open Patent Publication No. H2(1990)-225840, the center of gravity of the element is considered, but the inertia (or centrifugal force) around the center of gravity of the element and the weight of the element are not considered. Therefore, even though the center of gravity of each element is positioned radially outside of a respective pulley by enlarging the ears of the element, if the weight of the element increases, then the movement of the elements becomes unstable at the element-exit side of the driven pulley and at the element-entry side of the drive pulley, which locations are in the slack side of the metal V-belt. As a result, excessive loads against the metal rings (hereinafter referred to as "rings") and the elements are generated. In this condition, the transmission efficiency decreases over a range of high rotational speed (e.g., 5000-6000 rpm) because of the inertia of the element, and also, the life of the rings is shortened.
It is desirable that the above mentioned well-known method which is to improve the abrasion resistance and the transmission efficiency of the metal V-belt be revised to reduce the inertia of the element around the center of gravity of the element and to reduce the weight of the element for further improving the abrasion resistance and the transmission efficiency.